Conventionally, bicycle frames are of a diamond shape, comprising eight individual tubes, which could be made of metal or graphite fiber composites. The metal tubes are typically welded or brazed together, and the composite bicycle frames are typically formed either by securely joining the separate composite graphite tubes or by molding the tubes together as a one-piece, or unitary, frame.
A conventional diamond-shaped bicycle frame comprises a front triangle and a rear triangle. The front triangle comprises (a) a head tube for connecting to the bicycle handle bar and the front wheel assembly; (b) a seat tube having an upper portion connecting to the bicycle seat and a lower portion connected to the pedal support: (c) an upper tube connecting between the head tube and the upper portion of the seat tube: and (d) a lower tube connecting between the head tube and the portion of the seat tube.
On the other hand, the rear triangle of the conventional diamond-shaped frame comprises: (a) a left seat stay tube and (b) a right seat stay tube, both of which being provided in a rear upper fork comprising for connecting between the left upper portion of the seat tube and the left portion of the rear wheel assembly, and the right upper portion of the seat tube and the right portion of the rear wheel assembly, respectfully; and (c) a left chains stay tube and (d) a right chain staying tube, both of which being provided in a rear lower fork connecting between the left lower portion of the seat tube and the left portion of the rear wheel assembly, and the right lower portion of the seat tube and the right portion of the rear wheel assembly, respectfully.
Several methods have been taught in the prior art for making bicycle frames from individual thin-shelled graphite composite tubes. For example, U.S. Pat. No. 4,513,986 discloses a monocoque bicycle frame where the structural support is provided by a formed, stressed skin, in conjunction with internal stiffening ribs. U.S. Pat. No. 4,230,332 discloses another type of bicycle frame comprising a hollow panel extending between and secured to the tubular steering column and the tubular seat column replacing the hollow tubing ordinarily found in conventional bicycle frames. On the other hand, U.S. Pat. No. 4,479,662 discloses a bicycle frame using fittings that are generally hollow and have male assembly bearings with a decreasing taper section up to their ends and a larger section which is smaller than the section of the branch connections that are lengthened by the bearings. The corresponding ends of the tubes of the frame are secured adhesively by adapting them around the bearings the ends being formed with inside bearings with an increasing taper section up to the end corresponding to the male assembly bearings of the fittings. All these methods share a common disadvantage that, before all the tubes are joined together, they must be made individually. This involves not only long manufacturing time but also unnecessarily high production cost. In these methods, each and every element must be prepregged, and careful considerations must be employed for each tube regarding wrapping angle, overlapping style, patching, curing, molding, etc.
The so-called unitary bicycle frames eliminate many of the problems associated with having to adjoin individual tubes discussed above in making bicycle frames. These unitary bicycle frames are taught in U.S. Pat. Nos. 4,850,607; 4,889,355; 4,902,458; 4,941,674; 4,982,975; and 4,986,949, the content thereof is expressly incorporated by reference. In the manufacturing of the unitary bicycle frames, no welding or adhesion is required, and the finished bicycle frame does not have abutting joints. The unitary bicycle frames, which are made of composite fibers, are also substantially lighter than metal frames. The unitary bicycle frames involve the disadvantages that they require relatively complicated molds for manufacturing, and the thickness of the finished products becomes too large resulting in undesirably heavy weight of the finished bicycle frame. Furthermore, because high molding precision is required, the molds are expensive to construct. The unitary molding process involves three bulky mold members, which often could be misaligned, causing errors in the straightness of the finished bicycle frame.
All the conventional diamond-shaped frames, whether they be made by thin-shelled tubes joined together or by the unitary molding process, place great emphasis on providing torsional rigidity and the lateral bending rigidity, at the expense of increased vertical rigidity. In other words, in order to increase the torsional and lateral rigidities, the vertical rigidity was also increased drastically. As a result, the shock-absorbing ability was compromised and much of the impact force from the ground bumpings during riding was transmitted unabatedly to the rider. With the unitary bicycle frames, because of the large number of joints, tubes, and junctions involved, it is quite time consuming to prepare all the required overlappings, thus resulting in slow production rate and relatively high cost.